Heretofore, commercial aircraft, including the first generations of those propelled by jet engines, employed cable and pushrod connections between the flight control surfaces on the wings and tail of the aircraft and the control wheel, column, and pedals in the cockpit or flight deck. In some instances, these manual flight control systems were hydraulically boosted. However, these hydraulic systems were strictly auxiliary in nature, being employed only for such mundane tasks as raising landing gear. Thus, the failure of a hydraulic power supply system in such aircraft was of no serious consequence as the plane could continue to the next destination with the landing gear down (in some cases the gear could be raised with a manual handcrank).
With the advent of larger et aircraft such as the Boeing 747, fully powered operating systems for the aircraft's flight control surfaces became mandatory. Such airplanes are simply too large for a pilot with reasonable strength to fly and land safely without assistance. Thus, particularly in the case of larger aircraft, the status of hydraulic operating systems for the primary control surfaces has been elevated from auxiliary to primary. In fact, these hydraulic systems have become even more critical than the aircraft's engines because a safe landing can conceivably be made without engines whereas this is essentially impossible without hydraulic power.
An onboard supply of pressurized hydraulic fluid is required for the operation of an airplane's hydraulic systems. The criticality of hydraulic power supplies for the hydraulic systems of larger aircraft requires that these hydraulic power supplies be redundant. In addition, those systems must be easily serviced and maintained so that the airplane will not be delayed while mechanics attempt to diagnose and repair aircraft that are prepared for takeoff.
In aircraft with a flight crew of three, it is customarily the duty of the flight engineer to monitor the hydraulic systems and hydraulic power supplies of the aircraft during flight and to advise the aircraft pilots and ground personnel if faults occur. The latest generation of commercial airplanes, however, is designed to be operated by a crew of two; i.e., without a flight engineer. Thus, the functions, duties, and assignments of the flight engineer must be eliminated by automation or simplified so that they can be performed by the pilots without increasing flight crew duties to the extent that crew members are overburdened and can no longer operate the airplane safely.
The current approach to the automation of an aircraft's hydraulic power supplies involves the hard wiring of the appropriate components. Because the components of a particular system may be located on the flight deck and also at remote locations in the aircraft, and because these components typically each have their own separate logic, locating a fault can be a tedious and time consuming task. Furthermore, the "hard wiring" approach to the automation of onboard hydraulic power supplies is costly, both in terms of weight and money, because of the hardware and the extensive electrical wiring that is required.